/Engine/Source/Editor/UnrealEd/Classes/Editor/UnrealEdEngine.h
Will create a map of currently visible BSP surfaces.
virtual void CreateBSPVisibilityMap ( [UWorld](API\Runtime\Engine\Engine\UWorld) * InWorld, [TMap](API\Runtime\Core\Containers\TMap)< [AActor](API\Runtime\Engine\GameFramework\AActor) *, [TArray](API\Runtime\Core\Containers\TArray)< int32 >> & OutBSPMap, bool & bOutAllVisible )